Local anesthetics are drugs that block nerve conduction when applied locally to nerve tissue in appropriate concentrations. Generally, they act on any part of the nervous system and on every type of nerve fiber. Thus, a local anesthetic in contact with a nerve trunk can cause both sensory and motor paralysis in the area innervated. A preferred practical characteristic of compounds that are termed local anesthetics is that their action is reversible; i.e., that their use is followed by complete recovery of nerve function with no evidence of structural damage to nerve fibers or cells.
A suitable local anesthetic should combine several properties. It should not be irritating to the tissues to which it is applied, nor should it cause any permanent damage to nerve structure. Its systemic toxicity should be low because it is eventually absorbed from its site of application. The ideal local anesthetic must be effective regardless of whether it is injected into a tissue or applied locally to mucous membranes. It is usually important that the time required for the onset of anesthesia should be as short as possible. Furthermore, the action must last long enough to allow time for the contemplated surgery, yet not so long as to entail an extended period of recovery. Occasionally, a local anesthetic action lasting for days or even weeks or months is desirable, for example, in the control of chronic pain. Unfortunately, the available compounds employed for anesthesia for such long duration have high local toxicity. Neurolysis with slough and necrosis of surrounding tissues occur, and partial or complete transverse injury of the spinal cord with permanent paralysis may result if such a reaction occurs in the vicinity of the cord.
Local anesthetics prevent the generation and the conduction of the nerve impulse. Their main site of action is the cell membrane, since conduction block can be demonstrated in giant axons from which the axoplasm has been removed.
Local anesthetics block conduction by decreasing or preventing the enlarged transient increase in the permeability of excitable membranes to Na.sup.+ that is produced by a slight depolarization of the membrane (see Strichartz, G. R., and Ritchie, J. M., "The action of local anesthetics on ion channels of excitable tissues," in Local Anesthetics (Strichartz, G. R., ed.), Handbook of Experimental Pharmacology, Vol. 81, Springer-Verlag, Berlin, pp. 21-53 (1987)). This action of local anesthetics is due to their direct interaction with voltage-sensitive Na.sup.+ channels. As the anesthetic action progressively develops in a nerve, the threshold for electrical excitability gradually increases, the rate of rise of the action potential declines, impulse conduction slows, and the safety factor for conduction decreases. These factors decrease the probability of propagation of the action potential, and nerve conduction fails.
Although a variety of physicochemical models have been progosed to explain how local anesthetics achieve conduction block (see Courtney, K. R., and Strichartz, G. R., "Structural elements which determine local anesthetic activity," Id., pp. 53-94), it is now generally accepted that the major mechanism of action of these drugs involves their interaction with a specific binding site within the Na.sup.+ channel. However, whether or not all actions of local anesthetics are mediated by a common site remains unclear (Courtney and Strichartz, supra; Strichartz and Ritchie, supra).
Biochemical, biophysical, and molecular biological investigations during the past decade have led to a rapid expansion of knowledge about the Na.sup.+ channel and other voltage-sensitive ion channels (see Catterall, W. A., "Structure and function of voltage-sensitive ion channels," Science 242:50-61 (1988); Trimmer, J. S., and Agnew, W. S., "Molecular diversity of voltage-sensitive Na channels," Annu. Rev. Physiol. 51:401-418 (1989)). The Na.sup.+ channel of the mammalian brain is a heterotrimeric complex of glycosylated proteins with an aggregate molecular size in excess of 300,000 daltons; the individual subunits are designated .alpha. (260 kilodaltons), .beta..sub.1 (36 kilodaltons), and .beta..sub.2 (33 kilodaltons). After incorporation of the purified polypeptides into phospholipid vesicles, sodium flux into these vesicles occurs in response to veratridine, a substance known to cause persistent activation of Na.sup.+ channels. Only the .alpha. subunit is required to reconstitute channel function. Movement of Na.sup.+ into the vesicles is blocked by the neurotoxins tetrodotoxin and saxitoxin, and by local anesthetics (Tamkun et al., "The sodium channel from rat brain; reconstitution of neurotoxinactivated ion flux and scorpion toxin binding from purified components," J. Biol. Chem. 259:1676-1688 (1984)). The hydrophilic neurotoxins probably bind within the mouth of the channel which is formed by the .alpha. subunit. By use of a nonpermeant quaternary analog of lidocaine, it is possible to show that local anesthetics and tetrodotoxin interact at opposite ends of the Na.sup.+ channel (Rosenberg et al., "Reconstitution of neurotoxin-modulated ion transport by the voltage-regulated sodium channel isolated from the electroplax of Electrophorus electricus," Proc. Natl. Acad. Sci. USA 81:1239-1243 (1984)). A simple model of the Na.sup.+ channel in the plasma membrane is shown in FIG. 1.
It is well known that different nerve fiber types have varying relative susceptibilities to conduction block produced by local anesthetics. The various fiber types are generally classifiable into type A, type B, and type C. Generally, the greater the diameter of a given nerve fiber, the greater its speed of conduction. The larger axons are concerned with proprioceptive sensation and somatic motor function, while the smaller axons subserve temperature and pain sensation and autonomic function. In Table 1, the various fiber types are listed with their diameters, electrical characteristics, and functions.
TABLE 1 __________________________________________________________________________ Nerve Fiber Types in Mammalian Nerve Absolute Fiber Conduction Spike Refractory Diameter Velocity Duration Period Fiber Type Function (.mu.m) (ms) (ms) (ms) __________________________________________________________________________ A .alpha. Proprioception; 12-20 70-120 0.4-0.5 0.4-1 somatic motor .beta. Touch, pressure 5-12 30-70 .gamma. Motor to muscle 3-6 15-30 spindles .delta. Pain, temperature, 2-5 12-30 touch B Preganglionic &lt;3 3-15 1.2 1.2 autonomic C dorsal root Pain, reflex 0.4-1.2 0.5-2 2 2 responses sympathetic Postganglionic 0.3-1.3 0.7-2.3 2 2 sympathetics __________________________________________________________________________
In addition to variations in the speed of conduction and fiber diameter, the various classes of fibers in peripheral nerves differ in their sensitivity to hypoxia (Table 2). This fact has clinical as well as physiologic significance. Pressure on a nerve can cause loss of conduction in motor, touch, and pressure fibers while pain sensation remains relatively intact.
TABLE 2 ______________________________________ Relative Susceptibility of Mammalian A, B, and C Nerve Fibers to Conduction Block Produced By Various Agents Most Inter- Least Susceptible mediate Susceptible ______________________________________ Sensitivity to hypoxia B A C Sensitivity to pressure A B C ______________________________________
The differential rate of block exhibited by fibers of varying sizes is of great practical importance and may explain why local anesthetics affect the sensory functions of a nerve in a predictable order. Fortunately for the patient, the sensation of pain is usually the first modality to disappear; it is followed in turn by the sensations of cold, warmth, touch, and deep pressure, although individual variation is great.
The degree of block produced by a given concentration of local anesthetic depends markedly on how much and how recently the nerve has been stimulated. Thus, a resting nerve is much less sensitive to a local anesthetic than one that has been recently and repetitively stimulated; the higher the frequency of preceding stimulation, the greater is the degree of block obtained to a test shock. These frequency and use dependent effects of local anesthetics seemingly occur because the local anesthetic molecule in its protonated or charged form gains access to its binding site only when the Na.sup.+ channel is in an open state and because the local anesthetic may bind more tightly to and stabilize the inactivated state of the Na.sup.+ channel (see Courtney and Strichartz, supra). Measurements of single channel events show that voltage dependent activation is immediately followed by formation of an inactivated state. It is this transition that closes the channel (Aldrich et al., "A reinterpretation of mammalian sodium channel gating based on single channel recording," Nature 306:436-441 (1983)). Once bound, the local anesthetic greatly restricts the conformational changes in the sodium channel that underlie activation (Butterworth, J. F., and Strichartz, G. R., "Molecular mechanisms of local anesthesia: a review," Anesthesiology (1990, in press).
Veratridine is a classic activator of the voltage-gated Na.sup.+ channel. Veratridine binds to and selectively stabilizes an open conformation of the Na.sup.+ channel, leading to a persistent increase in Na.sup.+ permeability and a concomitant membrane depolarization. Veratridine binding and the associated depolarization are antagonized competitively by local anesthetics, and in principle, local anesthetic binding should be antagonized by veratridine.
Veratridine is a member of the veratrum alkaloids. The veratrum alkaloids constitute an abundant group of steroid-like polycyclic nitrogen-containing ring structures found in liliaceous plants. The alkaloids in two veratrum species, Veratrum album, Linn, and Veratrum viride, Aiton, and the species Schoenocaulon officinale, Gray, have been perhaps the best characterized, although as many as 20 different alkaloids have been identified to date (Krayer, O., and Acheson, G. M., Physiol. Rev. 26:336-446 (1946); Krayer, O., "Veratrum Alkaloids" in Pharmacology in Medicine, V. A. Drill (ed.), McGraw-Hill, N.Y. (1958)).
Veratridine, one of the major alkaloid components of veratrine, which is extracted from the seeds of Schoenocaulon officinale, the "Sabadilla seeds," has been widely used as a neuropharmacological tool to study the electrical properties of nerve and muscle fibers. Ulbricht, W., Rev. Physiol. 61:18-71 (1969); Ohta, M., et al., J. Pharmacol. Exp. Ther. 184:143-154 (1973); Catterall, W. A., Ann. Rev. Pharmacol. Toxicol. 20:15-43 (1980). Its ability to depolarize cells by altering the membrane-associated sodium channels is now well documented. Ulbricht, supra.
Aside from the well known use of veratridine as a sodium channel activator, veratridine has also been investigated as a potential hypotensive agent. Aviado, Domingo M., Pharmalogic Principles of Medical Practice, Williams & Wilkins, Baltimore, p. 548 (1972). Veratridine has also been reported as a selective inhibitor of angiotensin II receptors. Ball, D., et al., "Veratridine, Angiotensin Receptors and Aldosteronogenesis in Bovine Adrenal Glomerulosa Cells," Clin. and Exper. Theory and Practice A8(3):323-345 (1986).